1. Field of the Invention
The present invention relates to a projection apparatus that uses two digital micromirror devices (DMDs) and adopts light emitting diodes (LEDs) as the light source.
2. Descriptions of the Related Art
Currently, projection apparatuses have found a widespread use in offices, households, conference rooms and the like as imaging devices. Among various projection apparatuses, the digital light processing (DLP) projection apparatus, which applies both digital projection and display technologies, is capable of receiving a digital signal to generate a sequence of digital light pulses. The main concept of the DLP technology is to use a digital micromirror device (DMD) with a plurality of micromirrors, so that the light projected on these micromirrors will be reflected onto an imaging lens to form an image.
A projection apparatus can use three DMDs for each of the three primary colors to achieve a luminance of up to several thousands lumens and a high resolution. However, the expensive DMDs and more optical elements are required for achieving the desired effect tend to increase the cost of the product significantly. On the other hand, reducing the number of DMDs may reduce costs, but will degrade the luminance and resolution thereof. Therefore, a projection apparatus with two DMDs that have satisfactory optical efficiency has been developed in the art.
A conventional DLP projection apparatus comprising two DMDs is depicted in FIG. 1. The projection apparatus 1 mainly comprises a light source 11, a color wheel 14, a total internal reflection (TIR) prism 15, two DMDs 16a, 16b, a dichroic mirror 17, and a projection lens 18. The light source 11 is generally an ultra high pressure (UHP) mercury lamp. In more detail, light emitted from the light source 11 is converted into two kinds of colors by two kinds of segments of the color wheel 14. After entering the TIR prism 15, the light beams of two kinds of colors are further separated by the dichroic mirror 17 into three primary colors and projected onto the two DMDs 16a, 16b. Since the UHP mercury lamp has a poor red light spectrum, one DMD 16a is used for red color and the other DMD 16b is used for green and blue colors. Upon reflection from the DMDs 16a, 16b, the light beams are recombined and projected out of the projection lens 18 through the TIR prism 15.
However, a conventional DLP projection apparatus that employs a UHP mercury lamp as the light source 11 suffers from a number of disadvantages. Since the UHP mercury lamp relies on the excitation of mercury to generate ultraviolet light and thereby to provide a light source with high luminance, it takes a long time for the luminance to increase to the desired level. Furthermore, to generate light beams of the three primary colors, a color wheel has to be used in conjunction with the UHP mercury lamp, which may decrease the service life, be difficult to replace, decrease the color gamut, pollute the environment, consume power, and occupy a large space.
In summary, a conventional projection apparatus using two DMDs and an UHP mercury lamp is still deficient in performance, heat dissipation, and color gamut. In view of this, it is important to provide a projection apparatus that uses two DMDs and has a higher efficiency, wider color gamut, smaller profile, simpler structure, more inexpensive cost, and reduced starting time.